In producing copies of printed matter, for example, by an electrophotographic technique, a latent image is first formed on a light-sensitive material or a dielectric film, and then the latent image is developed by adhering particles, collectively referred to as toner, to the latent image, said toner comprising a binder resin with a colorant dispersed therein. Thereafter, the resulting toner image is transferred to a substrate such as paper and then fixed, for example, by heating. Although techniques such as a solvent method and an overcoating method are known for the fixation of the toner image, a heat-fixing method is commonly employed in which the toner image is heated to a temperature at which the toners become fluid and then is brought into contact with a substrate, thereby allowing the toners to be fixed to the substrate. In order to increase copying speed, a so-called heat roll fixing method, in which a substrate with the toner image transferred thereto is passed between a pair of rolls, i.e., a heating roll and a pressing roll, has been widely used as the heat-fixing method.
The heat roll fixing method has various advantages over the other heat-fixing methods. For example, heat efficiency is high, heat dissipation is reduced, and rapid fixation is possible. Thus, it is a very useful fixing method.
The heat roll fixing method, however, is liable to be subject to a phenomenon that part of the toner image sticks to the heating roll or pressing roll when it comes into contact therewith. This is referred to as an offset phenomenon. To prevent this offset phenomenon, a procedure has been employed in which the roll surface is covered with a material having superior releasing properties, such as a fluorine resin, and furthermore an offset-preventing liquid such as silicone oil is applied to the roll surface. This procedure enables prevention of the offset phenomenon to a certain extent, but suffers from various disadvantages. For example, (1) it requires the offset-preventing liquid, (2) it requires an additional apparatus for supplying the offset-preventing liquid to the roll surface in a constant amount, (3) there is a danger of oil leakage, and (4) the offset-preventing liquid tends to generate unpleasant odor on heating.
In recent years, therefore, many attempts have been made to perform heat roll fixation without the use of the offset-preventing liquid. One such method is described in Japanese Patent Pulbication No. 5549/78. Another method is to modify toners themselves so that it is not necessary to use the offset-preventing liquid. Such a method is described in Japanese Patent Application (OPI) Nos. 42354/74, 65231/74, 652d32/74, 107743/74, 11240/75, 27546/75, 28840/75, 81342/75, 85338/75, 93646/75, 93647/75, 134652/75, and 144446/75 (the term "OPI" as used herein means a "published uexamined Japanese patent application"). The method of preventing the offset phenomenon in the heat roll fixation by the modification of the toners themselves without the use of the offset-preventing liquid is satisfactory to prevent the offset phenomenon but is not always satisfactory to meet other properties required for dry toners. That is, among the individual toner particles the friction charge amount is greatly different, and as the result, there are incorporated various toner particles of from those having a small charge amount to those having a large charge amount, i.e., the toner chrage spectrum is extremely broad. Further, there may be caused incorporation of the desired toners and those having an opposite polarity thereto.
In order to overcome to the above problem, dry toners have been proposed in which as the biner resin a graft copolymer comprising (a) a crystalline polymer portion of at least one monomer selected from ethylene, propylene, and vinyl acetate, (b) an unsaturated polyester portion, and (c) a vinyl polymer portion, wherein (a) and (b) constitute a main chain and (c) constitutes a side chain, is used. These toners not only overcome the above-described problem, but also produce the advantages that fixation at low temperatures is possible and the temperature range in which fixation is possible can be extended. That is, they overcome many of the disadantages of conventional toners. The same is applied in U.S. Pat. No. 4,385,107.
It has been found, however, that although the toners referred to in the preceding are superior as negatively charged toners for use in development of positive electrostatic latent images, they are unsuitable as positively charged toners for use in development of negative electrostatic latent images, for example, of zinc oxide. This is believed to be due to the fact that the toners contain the noted unsaturated polyester portion.
As positively charged toners, toners prepared using a styrene-acrylic resin as a binder resin are generally suitable. Such toners, however, have a disadvantage in that their fixability at low temperatures is poor.
In order to render the toners containing the polyester portion positively chargeable, the present inventors made an attempt to incorporate a positive charge controlling agent such as ammonium salts, pyridinium salts, and azines. As the result, while positively chargeable toners can be obtained, there are caused blocking of the toners, reduction in powder fluidity of the toners, and marked increase in fixation temperature. This is believed to be due to the fact that the positive charge controlling agent incorporated is exposed on the surface by a bleeding phenomenon. Further, if these toners are mixed with carriers to prepare a developer, there is caused a so-called toner impaction phenomenon in which the carrier surfaces are contaminated by the toners.